Patterning of the face and skull involves coordinated signaling between the epithelial cells and skeletal-forming mesenchymal cells of the pharyngeal arches. Defects in epithelial-mesenchymal signaling within human arches result in a variety of birth defects, including facial deformities and cleft palate. Whereas major advances have been made in identifying the signaling pathways involved in facial development, how these pathways are integrated to generate precise patterns of skeleton in the head remains poorly understood. The long-term goal of this proposal is to understand the logic by which arch mesenchymal cells interpret diverse signals in their environment to develop into precisely shaped cartilages and bones. In the first round of funding, zebrafish genetics was used to identify a novel role for Jag1 Notch signaling in patterning the skeleton of the upper face. Mutations in Jag1 result in facial defects not only in zebrafish but also in humans with Alagille Syndrome, and hence these investigations have had direct implications for understanding how craniofacial development goes awry in this birth defect. It is currently thought that Notch signaling only goes in one direction, from the Jag1 ligand to the Notch receptor. In this renewal proposal, new data suggest that signaling may go in the other direction in the face, with Notch activating Jag1. As such, the genetic experiments proposed here may lead to a dramatic re-interpretation of the origins of facial and other organ defects in Alagille Syndrome. Moreover, as Notch signaling is widely utilized, the findings made here will have much broader implications for understanding how this pathway functions during animal development. A number of related Fox transcription factors are found in the precursor cells to the facial skeleton, with mutations in Foxc1 resulting in the craniofacial defects of Axenfeld-Rieger Syndrome. Whether other members of the Fox family also play specific roles in patterning the face remains unknown. In addition to Foxc1, our preliminary zebrafish studies suggest that Foxd2 and Foxf1 also have very specific roles in facial patterning. In this proposal, we use some of the most advanced genetic tools available in zebrafish to test a model in which Fox genes translate complex signals (such as Jag1-Notch) into the discrete shapes and positions of individual skeletal elements. Zebrafish is ideally suited for these studies as Fox expression is remarkably well conserved between mammals and fish, and powerful genetic and embryological tools in zebrafish allow us to dissect the precise roles of multiple Fox genes in regulating skeletogenesis. The completion of these Aims will inform how changes in bone and cartilage differentiation underlie the facial defects of Axenfeld-Rieger Syndrome.